The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Physicians use medical devices alone or in combination with drug therapies to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life.
Implantable medical devices are commonly used today to treat patients suffering from various ailments Implantable medical devices can be used to treat any number of conditions such as pain, incontinence, movement disorders such as epilepsy and Parkinson's disease, and sleep apnea. Additional therapies appear promising to treat a variety of physiological, psychological, and emotional conditions. As the number of implantable medical device therapies has expanded, greater demands have been placed on the implantable medical device.
One type of implantable medical device is an Implantable Neuro Stimulator (INS). The INS delivers mild electrical impulses to neural tissue using an electrical lead. The neurostimulation targets desired neural tissue to treat the ailment of concern. For example, in the case of pain, electrical impulses (which are felt as tingling) may be directed to cover the specific sites where the patient is feeling pain. Neurostimulation can give patients effective pain relief and can reduce or eliminate the need for repeat surgeries and the need for pain medications.
Implantable medical devices such as neurostimulation systems may be partially implantable where a battery source is worn outside the patient's body. This system requires a coil and/or an antenna to be placed on the patient's skin over the site of the receiver to provide energy and/or control to the implanted device. Typically, the medical device is totally implantable where the battery is part of the implanted device. The physician and patient may control the implanted system using an external programmer. Such totally implantable systems include, for example, the Itrel® 3 brand neurostimulator sold by Medtronic, Inc. of Minneapolis, Minn.
In the case of an INS, for example, the system generally includes an implantable neurostimulator (INS) (also known as an implantable pulse generator (IPG)), external programmer(s), and electrical lead(s). The INS is typically implanted near the abdomen of the patient. The lead is a small medical wire with special insulation. It is implanted next to the spinal cord through a needle and contains a set of electrodes (small electrical contacts) through which electrical stimulation is delivered to the spinal cord. The lead is coupled to the INS via an implanted extension cable. The INS can be powered by an internal source such as a battery or by an external source such as a radio frequency transmitter. The INS contains electronics to send precise, electrical pulses to the spinal cord, brain, or neural tissue to provide the desired treatment therapy. The external programmer is a hand-held device that allows the physician or patient to optimize the stimulation therapy delivered by the INS. The external programmer communicates with the INS using radio waves.
In totally implantable systems, it is desirable to efficiently and effectively recharge the implanted battery source. One such technique is the use of a replenishable, high capacitive power source as disclosed, for example, in U.S. Pat. No. 5,807,397. This reference is incorporated herein by reference in its entirety. These capacitive power sources require that they be replenished frequently, on a daily or other periodic basis, via an external, RF coupled device. This frequent replenishment of the capacitive power source, however, gives rise to a host of disadvantages, such as the frequent and continuous need for an available external, RF coupled device to replenish the capacitive power source, as well as the inconvenience of frequent replenishment of the capacitive power source.
An alternative rechargeable power source is a rechargeable battery, such as a lithium ion battery. The rechargeable battery provides a number of advantages over the capacitive power source including (1) less frequent replenishment; and (2) a smaller size and weight. Each of these advantages improves the quality of life of the patient. In this regard, U.S. Pat. No. 5,991,665 discloses an energy transfer system for transmitting energy to an implantable medical device. This reference is incorporated herein by reference in its entirety.
Although the rechargeable battery provides distinct advantages over the capacitive power source, care must be taken in the recharging process so as to maintain the battery's storage capacity and efficacy. For example, recharge of the battery at too high a voltage can damage the battery. Similarly, too high a recharge current can also be harmful to the battery. Damage may also result if the temperature of the INS increases significantly during recharge. Damage to the battery results in reduced efficacy, thereby requiring more frequent recharging, and even eventual replacement of the battery (by surgically explanting the INS). On the one hand, it is undesirable to overheat and damage the battery if it is charged too rapidly. On the other hand, however, it is desirable to charge the battery efficiently to minimize the charging time and improve patient quality of life.
Accordingly, it is desirable to provide a recharge management system for an implantable medical device that preserves the efficacy of the battery during recharge as well as provides efficient recharging of the battery.